This invention relates generally to electrostatographic imaging, and more particularly, it relates to a high-speed heat and pressure belt fusing apparatus for fixing images to a final substrate that exhibits long belt life, minimal edge wear and reliable stripping.
In a typical electrophotographic copying or printing process, a charge retentive surface such as a photoconductive member is charged to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive member is selectively exposed to light to dissipate the charges thereon in areas subjected to the light. This records an electrostatic latent image on the photoconductive member. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing one or more developer materials into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules either to a donor roll or to a latent electrostatic image on the photoconductive member. When attracted to a donor roll the toner particles are subsequently deposited on the latent electrostatic images. The toner powder image is then transferred from the photoconductive member to a final substrate. The toner particles forming the toner powder images are then subjected to a combination of heat and/or pressure to permanently affix the powder images to the copy substrate.
In order to fix permanently or fuse the toner material onto a substrate or support member such as plain paper by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers and/or into the pores of the support member or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the final substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members, at least one of which is internally heated. During operation of a fusing system of this type, the substrate to which the toner images are electrostatically adhered is moved through a nip formed between the pressure engaged rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. In a Nip Forming Fuser Roll (NFFR), the heated fuser roll is provided with a layer or layers that are deformable (i.e. conformable) by a harder pressure roll when the two rolls are pressure engaged. The length of the nip determines the dwell time or time that the toner particles remain in contact with the surface of the heated roll, the dwell time being also determinative of the fuser""s speed.
The layer or layers usually comprise an abhesive (low surface energy) material for preventing toner offset to the fuser member. Three materials, which are commonly used for such purposes, are fluoropolymers, fluoroelastomers and silicone rubber.
Roll fusers work well for fusing color images at lower speeds since the required process conditions such as temperature, pressure and dwell can be achieved. When process speeds approach faster speeds, for example 100 pages per minute (ppm), roll fusing performance is no longer acceptable. As fusing speed increases, dwell time must be maintained above a minimum, which means an increase in nip length. Increasing the nip length can be accomplished either by increasing the fuser roll rubber thickness, and/or reducing the modulus and/or increasing the outside diameter of the roll. However, each of these solutions reach their maximum effectiveness at about 100 ppm. Specifically, for an internally heated fuser roll, the fuser roll deformable layer thickness is limited by the maximum temperature the material forming the layers can withstand, and the thermal gradient across the layer. The roll size also becomes a critical issue for reasons of space, weight, cost and substrate stripping therefrom.
In order to obtain much higher fusing speeds than heretofore possible for color xerography, very large or long fusing nips are necessary. One way to achieve longer fusing nips for this purpose is to use a thick deformable belt instead of a fuser roll with a thick deformable layer or layers. Due to poor thermal conductivity, however, it is necessary to heat the outer surface of a thick elastomer belt over an extended contact zone using a source of thermal energy. To create a long, nip for extending fusing dwell time, it is desired that the belt be as thick as possible. However, belt flexibility can be compromised with relatively large belt thicknesses. Additional nip length can also be obtained using an elastomeric layer or layers on a pressure roll that contact the internal surface of the thick belt. The thicknesses of the elastomers on the pressure roll and the fuser belt along with other characteristics of the elastomers such as Shore A hardness contribute to the desired characteristics of the fusing nip. The thickness and the durometer of both elastomers can be varied to obtain the desired dwell times in the fusing nip.
One problem with a belt and roll arrangement that yields the desired nip length and thus the desired higher fusing speeds is that the creep is so low that substrate stripping from the belt without a separate stripping device is impossible. Creep is defined as the % velocity difference of the fuser belt surface in the fusing nip compared to its speed outside the nip.
Therefore, it is desired to provide a combination high-speed (i.e. above 100 ppm) belt and roll fuser for fusing color toner images that exhibit high gloss with minimal edge wear and long belt life and reliable substrate stripping.
Following is a discussion of references that may bear on the patentability of the present invention. In addition to possibly having some relevance to the question of patentability, these references, together with the detailed description of the present invention to follow, may provide a better understanding of the invention. The references that are discussed herein are hereby incorporated by reference in their entirety.
U.S. patent application Ser. No. 10/093,263 filed on Mar. 8, 2002, assigned to the same assignee as the present invention discloses a heat and pressure belt fuser structure having an endless belt and a pair of pressure engageable members between which the endless belt is sandwiched for forming a fusing nip through which substrates carrying toner images pass with the toner images contacting an outer surface of the endless belt, at least one of the pressure engageable members has one or more deformable layers, and the endless belt has a thickness of from about 1 to about 8 mm; and the fuser structure includes an external source of thermal energy for elevating a pre-nip area of the belt. The thick belts in combination with a deformable layer of at least one of the pressure member(s) cooperate to provide a large nip and adequate creep for intrinsic paper stripping. A creep value less than a predetermined value prevents stripping.
U.S. Pat. No. 5,890,047 granted to Rabin Moser on Mar. 30, 1999 discloses a combination belt and roll fuser has a pair of pressure engageable rolls with a belt looped or wrapped around one of the pressure engageable rolls such that the belt is sandwiched between the two rolls. The belt is deformed due to the force exerted by the pressure rolls such that it forms a single fusing nip. Substrates carrying toner images pass through the single fusing nip with the toner images contacting the outer surface of the belt. An internally heated, thermally conductive roll contacts a portion of the belt externally at a pre-nip location for elevating its temperature of the belt. The pressure engageable roll about which the belt is entrained is internally heated during warm-up for minimizing a phenomenon known as droop. This belt and roll fuser configuration exhibits the characteristics of a Nip Forming Fuser Roll (NFFR) fuser as discussed above.
U.S. Pat. No. 6,088,565 granted to Jia et al on Jul. 11, 2000 discloses a transfuse system that discusses the concept of fuser belt creep and states that the preferred creep is greater than 4%.
U.S. Pat. No. 6,246,858 discloses an electrostatographic reproduction machine that includes a fuser belt moving or position changing mechanism for moving the fuser belt and controllably changing its position axially relative to a plurality of rollers supporting the belt for movement in an endless path. The belt moving mechanism is suitable for controllably moving the endless fusing belt axially, (relative to the plurality of rollers) from a first fusing position to at least a second fusing position so as to reduce sheet edge wear in the same spot on the external fusing surface of the endless fusing belt.
U.S. Pat. No. 5,983,048 a temperature droop compensated NFFR fuser having a preheater structure which conveys the substrate carrying toner images past a radiant heat contained therein and then into the nip of a pair of pressure engaged fuser rollers that form the NFFR fuser. One of the fuser rollers is heated by an internal heater that is supplied a constant level of power that generally maintains the temperature of the heated roller to a temperature sufficient to fuse the toner images on the substrate. The preheater structure warms the substrate carrying toner images prior to entry into the nip of the fuser rollers to compensate for the temporary temperature droop of the fuser rollers that is encountered when the fuser moves from a standby mode to an operating mode. The combination of pre-warmed substrate and the temperature to which the heated fuser roller droops is sufficient to completely fuse the toner images on the substrate. With time in the operating mode, the fuser rollers recover from droop and the radiant heat source in the preheater structure is turned off.
U.S. Pat. No. 6,393,245 granted to Jia et al on May 21, 2002 relates to a transfuse system wherein stripping of substrates is assisted by the positioning of one guide roller supporting an intermediate transfuse belt relative to another guide roller.
U.S. Pat. No. 5,729,812 granted Mar. 17, 1998 relates to a combination dual hard roll and dual elastomeric belt fuser. A pair of hard, heated fuser rolls having elastomeric belts entrained thereabout are supported such that segments of the belts are sandwiched in a nip area therebetween. The belt segments are sufficiently thick to provide belt conformability resulting in high quality fused images. One of the belts is partially wrapped about one of the rigid rolls to form an extended heating zone and a combination heat and pressure zone through which substrates carrying toner images are moved.
U.S. Pat. No. 4,242,566 granted to Albert W. Scribner on Dec. 30, 1980 discloses a heat and pressure fusing apparatus that exhibits high thermal efficiency. The fusing apparatus comprises at least one pair of first and second oppositely driven pressure fixing feed rollers, each of the rollers having an outer layer of a thermal insulating material; first and second idler rollers, a first flexible endless belt disposed about the first idler roller and each of the first pressure feed rollers and a second flexible endless belt disposed about the second idler roller and each of the second pressure feed rollers, at least one of the belts having an outer surface formed of a thermal conductive material, wherein there is defined an area of contact between the outer surfaces of the first and second belts located between the first and second pressure feed rollers for passing the copy sheet between the two belts under pressure; and means spaced relative to the belt whose outer surface comprises the thermal conductive material for heating the outer surface thereof, whereby when an unfused copy sheet is passed through the area of contact between the two belts it is subject to sufficient heat and pressure to fuse developed toner images thereon.
U.S. Pat. No. 4,582,416 granted to Karz et al on Apr. 15, 1986 discloses a heat and pressure fusing apparatus for fixing toner images. The fusing apparatus is characterized by the separation of the heat and pressure functions such that the heat and pressure are effected at different locations on a thin flexible belt forming the toner-contacting surface. A pressure roll cooperates with a stationary mandrel to form a nip through which the belt and copy substrates pass simultaneously. The belt is heated such that by the time it passes through the nip its temperature together with the applied pressure is sufficient for fusing the toner images passing through.
U.S. Pat. No. 4,992,304 granted to Gilbert et al on May 1, 1990 discloses a fuser belt for a reproduction machine. The belt may have one of several configurations which all include ridges and interstices on the outer surface which contacts the print media. These interstices are formed between regularly spaced ridges, between randomly spaced particles, between knit threads. These interstices allow the free escape of steam from the media during high-temperature fusing of the reproduction process. As the steam escapes freely, the steam does not accumulate in the media causing media deformations and copy quality deterioration. Additionally, media handling is improved because the ridges and interstices reduce the unwanted but unavoidable introduction of thermal energy into the copy media.
U.S. Pat. No. 5,250,998 granted to Ueda et al on Oct. 5, 1993 discloses a toner image fixing device wherein there is provided an endless belt looped up around a heating roller and a conveyance roller, a pressure roller for pressing a sheet having a toner image onto the heating roller with the endless belt intervening between the pressure roller and the heating roller. A sensor is disposed inside the loop of the belt so as to come in contact with the heating roller, for detecting the temperature of the heating roller. The fixing temperature for the toner image is controlled on the basis of the temperature of the heating roller detected by the sensor. A first nip region is formed on a pressing portion located between the heating roller and the fixing roller. A second nip region is formed between the belt and the fixing roller, continuing from the first nip region but without contacting the heating roller.
U.S. Pat. No. 5,349,424 granted to Dalal et al on Sep. 20, 1994 discloses a heated, thick-walled, belt fuser for an electrophotographic printing machine. The belt is rotatably supported between a pair of rolls. One of the spans of the belt is in contact with a heating roll in the form of an aluminum roll with an internal heat source such as a quartz lamp. The belt is able to wrap a relatively large portion of the heating roll to increase the efficiency of the heat transfer. The second span of the belt forms an extended fusing nip with a pressure roll. The extended nip provides a greater dwell time for a sheet in the nip while allowing the fuser to operate at a greater speed. External heating enables a thick profile of the belt, which in turn allows the belt to be reinforced so as to operate at greater fusing pressures without degradation of the image. The thick profile and external heating of the belt also provides a much more robust design than conventional thin walled belt fusing systems.
U.S. Pat. No. 5,465,146 granted to Hgashi et al on Nov. 7, 1995 relates to a fixing device to be used in electrophotographic apparatus for providing a clear fixed image with no offset with use of no oil or the least amount of oil, wherein an endless fixing belt provided with a metal body having a release thin film thereon is stretched between a fixing roller having a elastic surface and a heating roller, a pressing roller is arranged to press the surface of the elastic fixing roller upwardly from the lower side thereof through the fixing belt to form a nip portion between the fixing belt and the pressing roller, a guide plate for unfixed image carrying support member is provided underneath the fixing belt, between the heating roller and the nip portion, to form substantially a linear heating path between the guide plate and the fixing belt, and the metal body of the fixing belt has a heat capacity per cm2 within the range of 0.001 to 0.02 cal/xc2x0 C.
The present invention provides a high speed heat and pressure belt and roll fuser structure comprising: a plurality of members including a deformable (i.e. conformable) endless belt and a pair of pressure rolls between which the endless belt is sandwiched for forming a fusing nip through which substrates carrying toner images pass with the toner images contacting an outer surface of the endless belt. Thus, one of the pressure members is positioned internally of the endless belt while the other one is positioned externally thereof. The internal pressure member comprises at least one deformable (i.e. conformable) layer and the belt comprises at least one deformable (i.e. conformable) outer layer.
An external source of thermal energy is provided for elevating a pre-nip area of the belt. The fuser of the present invention provides a high speed fuser with inherent glossing, minimal edge wear, long belt life and reliable substrate stripping the latter of which is provided through the interaction a post-nip portion of the belt with the external pressure roll contacting the outer surface of the belt.
The thicknesses as well as other characteristics such as ShoreA hardness on the internal pressure member and the deformable layer(s) of the belt are such that high-speed color fusing as discussed above is enabled. To this end, the aforementioned layers are sandwiched between the two pressure engageable members to provide a fusing nip that is sufficiently long to provide the desired high speed fusing. With such a nip a very low creep is inherent, creep being defined as the % velocity difference of the fuser belt surface in the fusing nip compared to its speed outside the nip or in the post-nip area. Substrate stripping presents a problem with such a configuration. Thus, in order to effect self-stripping, a portion of the belt is partially wrapped, in the post-nip region, around the external pressure roll engaging the external surface of the belt. The result of such post-nip wrapping is to compress the surface of the belt in that area thus decreasing the belt speed compared to a non-wrapped belt or an IPR wrapped belt in the post-nip area so that there is sufficient creep to effect stripping of imaged substrates. One of the rolls supporting the belt for movement in an endless path is positioned relative to the external pressure roll such that the post-nip wrapping is accomplished.